WATER TREE

Why and When do Cables Fault?After some years, electrical breakdowns caused by “Water Trees” occur.

Water trees are tiny (usually less than 0.025mm/(0.001 inches) tree-shaped micro-voids that develop in polymeric (XLPE, etc.) insulation under the influence of moisture and the electrical field.

Aging due to water treeing significantly reduces the electrical breakdown strength of the insulation.

Over time, the increasing presence of water trees seriously degrades the dielectric performance of the solid dielectric cable.

Causes of Water Intrusion

Manufacturing processStorageInstallationDig-in or failureDiffusionTermination leaksFlooding

Vented Water Tree

(Shown here with Electrical Tree) Vented water trees are generally initiated at protrusions on the interfaces between the dielectrics and conducting materials.

The visible tubules in the water trees are tracks which have been oxidized to form water-filled channels.

Bow Tie Water Tree

Bow Tie water trees usually initiate inside the bulk of the dielectrics at voids or contaminations.

Halo Water Tree

Halos are caused by water Halos are caused by water buildup from load and buildup from load and thermal cycling.thermal cycling.

Technology Extension to EPR

AC breakdown strength and impulse breakdown strength increases to values better than for new EPR cables.

Reliant Energy and EPRI performed work and reported at IEEE/PES winter meeting in Singapore (January 2000).

Water-TreeWater-Tree

General Cable General Cable PhotoPhoto

Extremely Large Electrical Tree

Carbon – the initial formation Of the electrical tree

Electrical Tree

Electrical trees develop from untreated water trees and will cause failure in days or weeks.

All Pre-1980 Vintage PE Cables are in Poor Shape.

Typical Perform ance of pre-1980 vintage XLPE Cables

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AC Breakdown (v/mil)

Typical XLPEVPC ControlEGD ControlEPRI XLPE

“The [AC breakdown] values obtained were at best 1/3 of the level generally regarded as normal for … new cable (800 V/mil).”Source: Characterization of Failed Solid-Dielectric Cables: Phase 2, EPRI EL-5387, project 1782-1, Final report September 1987.

Cables with less than 400 v/mil are “poor”

After the first decade, the degradation slope makes it difficult to distinguish “poor” from “bad”

“Treeing” is where the Fault LiesTreeing is the most common cause of dielectric cable failure. The overall result is the generation of a cavity which can sustain partial discharge, creating an “Electrical Tree”. Untreated, water trees become electrical trees and cause cable failure.

Aging cables can remain a valuable asset for utilities. With proven dielectric enhancement technology available, wholesale cable replacement is an unnecessary cost option. At a fraction of replacement cost, any prudent planner should consider treatment with CableCURE.Both replacement and treatment yield highly reliable systems, however, the capital cost for injection is typically two to three times less than cable replacement.

Fault as a Result of Fault as a Result of TreeingTreeing